The present invention relates to an apparatus and method for use with a web and more specifically for cooling a web of printed paper.
A printing press applies ink to a web of paper as the web is moved lengthwise through the printing press. The freshly printed web is moved through a dryer and is then moved through a chill roll system in order to cool the heated web and to set the ink. The chill roll system includes a succession of chill rolls which are cooled by water circulating through the interior of the rolls.
As the paper web moves from the dryer to the chill roll system, boundary layers of air adhere to the opposite side surfaces of the moving web and are carried along with the moving web. Vaporized chemical solvents and ink residue become trapped in the boundary layers on the surfaces of the moving web as the web emerges from the dryer. The chemical solvents and ink residue are carried by the boundary layers toward the chill roll system.
The boundary layers adhering to the web surfaces are carried around the rolls in the chill roll system. The boundary layers insulate the heated surfaces of the web from the cooled surfaces of the chill rolls and inhibit heat transfer from the web to the chill rolls. Furthermore, the vaporized ink and chemical solvents in the boundary layers are deposited as residue on chill roll surfaces as the web moves through the chill roll system. Ink residue accumulating on the chill roll surfaces may be transferred back onto the web and soil the printed web surfaces which follow.
A known chill roll system for transferring heat from a web is disclosed in U.S. Pat. No. 4,476,636. This chill roll system includes a pair of small boundary layer control rolls which are disposed in sliding contact with the opposite side surfaces of the moving paper web. Each boundary layer control roll rotates in a direction opposite to the direction of the web sliding over the roll. A boundary layer carried on the web surface is squeezed off as the web slides against the boundary layer control roll.
Another known chill roll system for transferring heat from a web is disclosed in U.S. Pat. No. 5,036,600. This chill roll system includes a plurality of chill rolls which are closely spaced to define zones of interference. A boundary layer on the web sections moves through the zones of interference and is scrubbed away from the web.
Another known apparatus 10 for use in transferring heat from a web is illustrated in FIGS. 1 and 2. The prior art apparatus 10 includes a cylindrical chill roll 12 which is rotated in a clockwise direction about its central axis by a suitable drive train (not shown). The chill roll 12 has a cylindrical outer side surface 14 which is cooled, in a known manner, by circulation of water through the chill roll. A web 16 of printed paper is conducted around the chill roll 12 and moves in the direction indicated by an arrow 18 in FIG. 1.
A linear nozzle 22 (FIGS. 1 and 2) extends across the web 16. The nozzle 22 directs a jet of air against a linear area which extends across the web 16 in a direction parallel to the central axis of the chill roll 12. The linear area where the flow of air from the nozzle 22 engages the web 16 is a short distance downstream from a line of tangential engagement of the web 16 with the chill roll. The linear area where air from the nozzle 22 is applied against the web 16 extends parallel to the line of tangential engagement of the web with the chill roll 12.
With the prior art apparatus illustrated in FIGS. 1 and 2, it has been noted that on occasion there are hot strips on the web exiting from the chill roll 12. These hot strips may be at a temperature which is as much as 100.degree. F. above the temperature of the surrounding areas. The hot strips tend to occur more often and with greater severity at higher web speeds (speeds greater than 1,500 feet per minute).
The hot strips or low heat transfer areas on the web 16 are also areas where solvent vapor condenses. The difference in solvent condensation may be more than 100 times greater at the hot strips or areas of low heat transfer than in the relatively cool or cold areas. It is theorized that the hot strips or areas of low heat transfer are due to fluid (vapor or liquid) pockets being formed between the web and the chill roll.